This invention relates to liquid varnish coating compositions having a low or nil solvent content in which the binder comprises a glycidyl ester and a polyamine having at least three active hydrogen atoms covalently linked to nitrogen atoms.
It is known to cure polyepoxides with anhydrides or polyamines to obtain valuable thermosets used predominantly as coating and molding compounds. The curing of polyepoxides with anhydrides, widely used for the production of molding compounds, has no practical significance for varnishes and coatings, since in spite of a high curing temperature (starting with 120.degree. C.) the processing time at room temperature (pot life) of such varnishes is too brief and the thus-obtained coatings are too brittle. In contrast thereto, the curing of polyepoxides with polyamines and/or polyamidoamines is of great significance for the production of varnishes and coatings; this curing process takes place at room temperature or a slightly elevated temperature (up to about 80.degree. C.). Such cold-setting varnishes are of special interest for the coating of large, bulky articles, e.g. railroad boxcars or bridges, as well as for the coating of heat-sensitive substrates, e.g. wood or synthetic resins which, due to their size or heat sensitivity, cannot be provided with customary baking enamels curing at above 100.degree. C.
Since no cleavage products are liberated during the curing reaction of epoxy groups with amines, it is possible to employ varnishes based on polyepoxides and amines as a binder for the production of thick-layer varnish coats in one operating step, if they can be applied with a low solvent content or even solvent-free. Such low-solvent varnishes which are cured in an additive reaction have the advantages of special economy as well as of being ecologically acceptable. The preparation of low-solvent content or solvent-free varnishes on the basis of polyepoxides and amines is known. In most cases, bisglycidyl ethers are utilized as the polyepoxides in this connection, obtainable by the reaction of 2,2-bis(4-hydroxyphenyl)propane (bisphenol A) and epichlorohydrin. Depending on how this reaction is conducted, the bisphenol A-bisglycidyl ethers are obtained in the liquid phase or in the solid, higher-molecular weight phase. Solid bisphenol A-bisglycidyl ethers can be processed into solvent-free powder varnishes in conjunction with particularly low-reactive amines, e.g. dicyandiamide. In addition to being inexpensive in manufacture and application, which makes such varnishes unusable for conventionally equipped and designed varnish manufacturers and varnish plants working with liquid varnishes, these powder varnishes have the disadvantage that they must be baked in at temperatures of above 120.degree. C.
The utilization of liquid bisphenol A-bisglycidyl ether and reactive amines, e.g. aliphatic and cycloaliphatic amines, especially with the concomitant use of reactive thinners, e.g. monoglycidyl ethers and/or bisglycidyl ethers of alcohols, makes it possible to produce liquid, conventionally obtainable and applicable, low-solvent content varnishes which are cured at room temperature or at temperatures of up to about 80.degree. C. A serious disadvantage of the otherwise excellent coatings made from these varnishes is their lack of weathering resistance, manifesting itself by loss of luster (chalking) and yellowing. Such coatings furthermore tend to become brittle. With the use of polyaminoamides in place of amines as the curing agents, more elastic coatings are obtained, but a greater amount of solvent is needed for the application of such varnishes due to the high viscosity of the polyaminoamides; thus, these varnishes are less favorable for the environment.
On the basis of the coating characteristics displayed by bisphenol A-bisglycidyl ether/amine varnishes, these are well suitable for use as primers, but are unsuitable for decorative cover coats exposed to outside weather conditions.
More weatherable coatings can be obtained on the basis of epoxidized cycloaliphatic olefins ("olefin oxides") and amines, but such varnishes cannot presently be cured at temperatures of below 100.degree. C.
The production of more weatherproof, cold-set coatings is possible on the basis of bisglycidyl esters of dicarboxylic acids. The best properties are attained with bisglycidyl esters of cycloaliphatic dicarboxylic acids, e.g. hexahydrophthalic acid bisglycidyl ester. Corresponding coatings exhibit satisfactory hardness, good elasticity upon gradual stress (Erichsen depression), but insufficient elasticity upon sudden deformation (impact depression). A further disadvantage of such varnishes is the very short pot life, probably due to the high epoxy group and amine group concentration present in the binder. The weatherability of such coatings is markedly improved as compared to bisphenol A-bisglycidyl ether/amine coatings, but still reaches only a medium level. This can be due perhaps also the high nitrogen content in the coating, which is necessary for optimum curing.
The production of nonpolluting, low-solvent content varnishes which, when cured at room temperature or temperatures of up to 80.degree. C., yield coatings of good weatherability and hardness, as well as excellent elasticity, is thus impossible with the use of the conventional epoxy resins.
Numerous prior art teachings relate to polyesters which carry glycidyl groups. For example, polyesterpolycarboxylic acids are used for preliminary extension and hardening of polyepoxides, such as bisphenol A-diglycidyl ethers or dicarboxylic acid-diglycidyl esters, in order to obtain elastic cross-linking products. The polyester-polycarboxylic acids used for the preparation of such products are prepared mostly from a diol and an aliphatic dicarboxylic acid, e.g. 1,6-hexanediol and sebacic acid in a molar ratio of 4:5; see DOS (German Unexamined Laid-Open Application) No. 1,720,427. The process of rendering commercial epoxy resins elastic, however, results in a markedly lower stability with respect to water and chemicals. Utilization of the very highly viscous, preliminarily extended epoxy resins takes place during the preparation of casting resins in the melt. Due to the large amount of solvent required, such epoxy resins are unsuitable for the nonpolluting production of varnishes.
Polyglycidyl esters obtained by reacting polyesterpolycarboxylic acids with epichlorohydrin are hardly known, at least on a commercial scale. Although German Pat. No. 1,009,590 describes the reaction of a polyester of adipic acid and butanediol (molar ratio 3:2) with epichlorohydrin by way of the potassium salt, glycidylation takes place only to a very incomplete extent. In case of a more comprehensive reaction with epichlorohydrin, a glycidyl ester is obtained which, when cured with hexahydrophthalic anhydride, yields varnish films which are too soft. In British Pat. No. 884,033 (Example 39), a mixture of an unspecified polyester and phthalic anhydride is reacted with epichlorohydrin in the presence of a basic ion exchanger, thus obtaining an epoxide mixture with a high chlorine content which consequently is unsuitable as a varnish binder.
Furthermore, glycidyl esters with 2-6 epoxide equivalents per mole are conventional which have been obtained from low-molecular weight partial esters carrying carboxyl groups (mostly of a degree of polycondensation = 3) by reacting such esters with epichlorohydrin and an alkali in the presence of catalysts, e.g. see German Pat. No. 1,165,030; British Pat. No. 884,033; DOS No. 1,816,933; DOS No. 1,916,287; British Pat. No. 1,026,141; and DOS No. 1,643,789.
These acidic partial esters are produced by the esterification of one mole of a polyalcohol or polyether dialcohol (see, for example, German Pat. No. 1,904,110) with n = 2-6 hydroxy groups and n (in most cases 2) moles of an aromatic, cycloaliphatic or aliphatic dicarboxylic acid and/or the anhydrides thereof. In this process, the hydroxy groups are partially or entirely esterified. As can be seen from comparative experiments, these conventional glycidyl esters, when cured with polyamines, yeild coatings with unsatisfactory mechanical properties; the same holds true for amine-cured coatings of bisglycidyl esters of aliphatic, aromatic and cycloaliphatic dicarboxylic acids, e.g. adipic acid bisglycidyl ester, isophthalic acid bisglycidyl ester or hexahydrophthalic acid bisglycidyl ester.